The present invention relates to a dielectric ceramic composition and dielectric resonator which stably control dielectric constant ∈r, resonator acutance Q value and temperature coefficient xcfx84f in a high frequency region such as of microwave and millimeter-wave, and which have less variation in these characteristic values. More particularly, the invention relates to a dielectric ceramic composition, a method for preparing a dielectric ceramic material and a dielectric resonator, which are applied for high frequency regions in various resonator materials, dielectric substrate materials for MIC (monolithic IC) ,dielectric waveguide materials, multilayer ceramic capacitors, and the like.
Dielectric ceramic materials are widely utilized in resonator materials, MIC dielectric substrate materials, dielectric waveguides, dielectric antennas, and any of various electronic components, which are employed for microwave and millimeter-wave regions in various equipment such as car telephones, cordless telephones, personal wireless units, and satellite broadcasting receivers. The dielectric ceramic materials are generally required to have the following three principal characteristics:
(i) High dielectric constant to meet a demand for size reduction of devices, because the wavelength of propagating electromagnetic wave is reduced to 1/∈r1/2 in dielectrics;
(ii) A small dielectric loss in high frequency regions, that is, a high Q value; and
(iii) A small change in resonant frequency with respect to a temperature change, that is, a stably low temperature dependency of dielectric constant ∈r.
Exemplary dielectric ceramic materials hitherto known are oxide ceramic materials such as BaOxe2x80x94TiO2 based materials, BaOxe2x80x94REOxe2x80x94TiO2 (wherein REO is an oxide of a rare earth element) based materials, and MgTiO3xe2x80x94CaTiO3 based materials (see Japanese Unexamined Patent Publication Nos. 61-10806/1986 and 63-100058/1988).
Although the BaOxe2x80x94TiO2 based materials have a high dielectric constant ∈r of 37 to 40, and a high Q value on the order of 40,000, it is difficult to attain a resonance frequency temperature coefficient xcfx84f of zero in a single phase, and these materials suffer from a great change in the dielectric constant to a composition change, and in the temperature dependency of dielectric constant. Therefore, it is difficult to stably reduce resonance frequency temperature coefficient xcfx84f, while maintaining a high dielectric constant and a low dielectric loss.
Known as the BaOxe2x80x94REOxe2x80x94TiO2 based materials are BaOxe2x80x94Nd2O3xe2x80x94TiO2 based materials and BaOxe2x80x94Sm2O3xe2x80x94TiO2 based materials. These materials, however, have an extremely high dielectric constant ∈r of 50 to 80. Some of them have a resonance frequency temperature coefficient xcfx84f of zero, however, they have a low Q value on the order of 5,000.
As to the MgTiO3xe2x80x94CaTiO3 based materials, Q value is large on the order of 30,000. Some of them have a resonance frequency temperature coefficient xcfx84f of zero, however, their dielectric constants are small of 16 to 25.
Accordingly, any of the above materials does not satisfy the three characteristics (i) to (iii) at the same time, which are required of dielectric materials for high frequency application. Although these materials provide a high Q value in a high frequency region, Q value at a high temperature (120xc2x0 C.) is reduced considerably than Q value at room temperature (25xc2x0 C.). Hence, they fail to satisfactorily enjoy the merit of high Q value, thus causing the problem of increasing the insertion loss (IL) of a resonator.
On the other hand, the present applicant has proposed a dielectric ceramic composition which comprises a complex oxide containing a rare earth element, Al, Ca and Ti (see Japanese Unexamined Publication No. 6-76633/1994). This dielectric ceramic composition has excellent features that it has a high dielectric constant of 34 to 46, and a high Q value of over 20,000. On the contrary, during its manufacturing process there is a considerable variation in dielectric constant ∈r, Q value, and resonance frequency temperature coefficient xcfx84f. This makes it difficult to stably control these characteristics.
The above dielectric ceramic composition also has the problem that the rate of retention of Q value at a high temperature (120xc2x0 C.) to Q value at room temperature (25xc2x0 C.) is insufficient. With the development of communication industry, there is a demand for materials having a higher Q value.
It is a main object of the present invention to provide a dielectric ceramic composition, a method for preparing a dielectric ceramic material, and a dielectric resonator, which have a large dielectric constant ∈r, high Q value and low resonance frequency temperature dependency, and which have less variation in dielectric constant ∈r, Q value and resonance frequency temperature coefficient xcfx84f, thereby to stably control these characteristic values.
It is another object of the invention to provide a dielectric ceramic composition for high frequency application which can maintain a high retention of Q value at 120xc2x0 C., to Q value at room temperature (25xc2x0 C.) , as well as a dielectric resonator using this composition.
According to the invention, there is provided a dielectric ceramic composition wherein a main crystal phase is a perovskite-type crystal phase. This composition comprises a complex oxide which contains, as a metal element, at least a rare earth element (Ln) ; Al; Sr, alternatively, Sr and Ca; and Ti, and which is represented at a mole ratio by the following composition formula:
aLn2OxbAl2O3cMOdBaOeTiO2xe2x80x83xe2x80x83(1)
wherein M is Sr, alternatively, Sr and Ca; and 3xe2x89xa6xxe2x89xa64, a, b, c, d and e satisfying the following equations:
0.056xe2x89xa6axe2x89xa60.450;
0.056xe2x89xa6bxe2x89xa60.450;
0.100xe2x89xa6cxe2x89xa60.500;
0xe2x89xa6dxe2x89xa60.100;
0.100 less than e less than 0.470;
0.75 xe2x89xa6b/axe2x89xa61.25;
0.75xe2x89xa6e/(c+d)xe2x89xa61.25; and
a+b+c+d+e=1
This composition enables to obtain a large dielectric constant ∈r and high Q value, and to lessen variations in dielectric constant ∈r, Q value and resonance frequency temperature coefficient xcfx84f, thereby to provide a dielectric ceramic material capable of stably controlling these characteristic values. That is, the composition of the invention has accomplished from the point of view of that control of the main crystal phase in a dielectric ceramic composition is important in order to reduce and stably control the variation of dielectric characteristics.
The dielectric ceramic composition preferably comprises a complex oxide which contains, as a metal element, at least La, Al, Sr and Ti, and which is represented at a mole ratio by the following composition formula:
aLa2O3bAl2O3cSrOeTiO2xe2x80x83xe2x80x83(2)
wherein a, b, c and e satisfy the following equations:
0.1061xe2x89xa6axe2x89xa60.2162;
0.1050xe2x89xa6bxe2x89xa60.2086;
0.3040xe2x89xa6cxe2x89xa60.4185;
0.2747xe2x89xa6exe2x89xa60.4373;
0.75xe2x89xa6b/axe2x89xa61.25;
0.75xe2x89xa6e/cxe2x89xa61.25; and
a+b+c+e=1
By using La and Sr together, it is possible to form a solid solution of LaAlO3 and SrTiO3, thereby to improve Q value.
Preferably, a dielectric ceramic composition of the invention is substantially composed of a single crystal phase of a perovskite-type crystal phase. It is more preferable that the perovskite-type crystal phase be composed of a solid solution which comprises at least LnAlO(x+3)/2 (Ln is a rare earth element, and 3xe2x89xa6xxe2x89xa64) , and RTiO3 (R is an alkaline earth metal containing at least Sr).
Thus, a still higher Q value is obtainable by a dielectric ceramic composition comprising a complex oxide which contains, as a metal element, at least La, Al, Sr and Ti, and which has a solid solution of LaAlO3 and SrTiO3, as a main crystal phase.
A method of preparing a dielectric ceramic material of the invention is characterized in that a calcined powder of which main crystal phase is LnAlO(x+3)/2 (3xe2x89xa6xxe2x89xa64) , and a calcined powder of which main crystal phase is RTiO3 (R is an alkaline earth metal containing at least Sr) are blended, molded and fired.
With this method, the solid dissolution between the two calcined powder is facilitated, thereby to obtain a dielectric ceramic material of a single crystal phase which is substantially composed of a complex oxide crystal phase of a perovskite-type (ABO3 type).
The present invention also provides a dielectric ceramic composition which contains Mn, in terms of MnO2, in an amount of 0.01 to 3 parts by weight, to 100 parts by weight of a main composition represented by the composition formula (1), particularly, the composition formula (2).
This composition enables to stabilize Q value and to increase the rate of retention of Q value at a high temperature (120xc2x0 C.) to Q value at room temperature (25xc2x0 C.) , thereby to increase no-load Q of a resonator.
According to the present invention, a dielectric resonator be operated by electromagnetic coupling comprises a dielectric ceramic material and a pair of input and output terminals disposed on the sides of the dielectric ceramic material, and the dielectric material comprising the dielectric ceramic composition as described. This enables to provide a resonator having excellent stability of no-load Q and resonance frequency.
Other objects and advantages of the present invention will become apparent from the following detailed description.